In the previous activity, Spheres of Influence, students learned about emulsions by shaking up water in oil. In this lesson, students will be shaking oil and vinegar, which are the basis for most salad dressings. Shaking up this mixture breaks up its vinegar (which is water-based) into tiny droplets that are temporarily suspended in oil.

As students will see in this activity, mayonnaise is nothing more than ordinary salad dressing with the addition of an emulsifying agent (egg yolk) to make the mixture more permanent.

Be sure to visit Vicki's Kids' Science Page for more great science fun, a complete list of her books, and information about how you can invite Vicki to come to your school. And don't miss her library of science videos too. Or visit Vicki and other great authors of nonfiction for children at the INK Think Tank.

I've set up this activity as a demonstration; but this is a fun activity for students to do in small groups. They will be making mayonnaise -- and coming to understand its chemistry as they do it.

The Plot

Act I

Crack the raw egg into the jar.

Add three tablespoons of oil.

Tightly screw on the jar lid and shake at least fifteen times. (The mixture will be lemon yellow. The oil should not form a separate layer when you finish shaking.)

Measure a cup of oil. Pour a little oil into the jar. Tightly screw on the lid and shake fifteen times. What happens?

Repeat the above action several times. What happens? What changes do you observe?

When you have added half the oil from the measuring cup (equivalent to 1/2 cup), add two tablespoons of vinegar to the jar. Again, shake hard fifteen times.

Add two tablespoons of vinegar and shake hard two more times -- for a total of six tablespoons of vinegar in the recipe.

Continue by adding the rest of the oil, a little at a time. Shake after each addition. What happens?

Eventually, the mixture will become thick and creamy. Stop shaking it after all the oil is mixed in. To season the mixture, add a teaspoon of salt and half a teaspoon of pepper and stir well. You have mayonnaise!

Homemade mayonnaise is not as thick as mayonnaise from the store is. But some people think it tastes better. Keep it in the refrigerator. Cover it so a skin doesn't form on the surface of your mixture.

Behind the Scenes

Mayonnaise comprises tiny droplets of oil spread evenly through the egg yolk, which is the active emulsifying agent in eggs. Egg yolk coats the oil droplets as they form and prevents them from coming together and forming a separate layer. If you add the oil too fast, or if you add too much oil at a time, the droplets will come together before they can be forced into the egg yolks; the mayonnaise will curdle, or separate. If that happens, you can correct the situation by starting over with a fresh egg, but add the curdled mayonnaise to the egg rather than the other way around.

You can tell when the emulsion has formed because the mixture gets thick. This usually happens after about 1/3 cup of oil has been added.

The End

Several cookbooks make the claim that mayonnaise is difficult to make on a rainy day or when a thunderstorm is threatening. But mayonnaise is made commercially on every working day, regardless of the weather. You might want to set your student-scientists to work doing experiments to test that claim.

See additional experiments with food in Vicki Cobb's book Science Experiments You Can Eat (HarperCollins, 1994).